Associative thickeners are generally water-soluble or water-dispersible polymers bearing a small amount of hydrophobic groups. They are widely used to control rheology and application properties of a wide variety of aqueous systems. One major application area for associative thickeners is in water-borne coatings. It is well known that associative thickeners that are used as rheology modifiers in water-borne coatings, are difficult to prepare in the form of highly concentrated aqueous solutions with such a low viscosity so as to allow ease of pouring, pumping and dosing into aqueous systems such as water-borne protective coatings. Mechanism by which associative thickeners may thicken aqueous phase may involve hydrophobic associations between the hydrophobic groups in the thickener molecules and/or interaction of the associative thickeners with hydrophobic components present in the water-borne coatings compositions. The various types of associative thickeners are based on modified-natural polymers, such as cellulose ethers that have been hydrophobically-modified, synthetic polymers, such as hydrophobically-modified ethoxylated polyurethanes (HEURs), hydrophobically-modified alkali-soluble emulsions (HASEs) and hydrophobically-modified poly(acetal- or ketal-polyethers) (HM-PAPEs). The paint performance properties of water-borne coatings formulated with the associative thickeners vary in the most fundamental properties such as thickening efficiency, sag, leveling and film build (ICI viscosity).
Synthetic associative thickeners in recent years have gained an increased importance, particularly in controlling the rheology of water-borne coatings and latex paints. Since synthetic associative thickeners are prepared from basic chemicals, they can be prepared with certain properties in mind. In other words, from the ground up they can be tailored for the desired properties. Synthetic associative thickeners serve several roles in aqueous systems. In latex paints and water-borne coatings, for instance, the thickener provides improved stability and pigment suspension, and improved rheological and application properties. In personal care products, the thickeners provide improved body, feel, smoothness and richness of the product, making the product aesthetically more pleasing. In other industries, the synthetic associative thickeners are used to improve other desired properties.
Control of polymer solution viscosity and possibility to incorporate polymers in solution at high percent of solids content with relatively low viscosity are important requirements from a manufacturing and application standpoint. These requirements allow balancing overall cost of product use and achieving the proper procedures for product incorporation into the application formulation. Examples of these synthetic associative thickeners that are used as rheology modifiers in water-borne coatings include hydrophobically modified ethoxylated urethane (HEUR), hydrophobically modified alkali-soluble emulsion (HASE), hydrophobically modified polyethylene glycol (HM-PEG) and hydrophobically modified poly(acetal- or ketal-polyether) (HM-PAPE).
Because of intermolecular association through their hydrophobes, aqueous solutions of associative thickeners exhibit high viscosities even at lower concentrations. To suppress the solution viscosity of various types of associative thickeners, viscosity suppressing aids, such as propylene glycol, butyl carbitol, cyclodextrins and surfactants are used (see U.S. Pat. No. 6,063,857; U.S. Pat. No. 5,137,571; U.S. Pat. No. 5,425,806; U.S. Pat. No. 6,150,445; U.S. Pat. No. 5,378,756; U.S. Pat. No. 5,959,013; U.S. Pat. No. 5,574,127; U.S. Pat. No. 6,162,877). Typical useful viscosity ranges for these aqueous combinations of thickener and additive are about 1000 cps to 5000 cps. Typical desired polymer solid concentrations are in the range of about 15 to 30% by weight.
The HM-PAPE, manufactured and marketed by Hercules Incorporated under the trademark Aquaflow® are currently sold to customers in liquid form. Currently some of these products are sold as 25% active polymer solutions in a mixture of water and butyl carbitol.
The existing Aquaflow products are moderate to difficult to incorporate in a high gloss paint mix, having a styrene-acrylic binder. This problem also applies to solvent-free water-borne coatings with a pigment volume concentration (PVC) of 70% and containing a vinyl acetate/ethylene (VAc/E) latex.
From customer surveys, it was found that most latex paint manufacturers encountered difficulties while incorporating synthetic associative thickeners into a paint formula. Ideally, after the thickener is added to the base paint, the resulting paint mixture should be easy to stir and the thickener should manifest its thickening capacity very quickly, preferably within several minutes. In this way, the full viscosity of the paint is achieved fairly quickly and no further increase in viscosity occurs on storage.
The ease of incorporating thickener in latex paints is measured by recording the time the thickener starts to effectively thicken the paint formulation and by the time no further change in viscosity is observed. After incorporating the thickener into the base paint, the appearance of the paint should be smooth, meaning there should be no lumps left behind. The viscosity of the resulting paint is then measured. In the paint industry, this viscosity is referred to as Stormer viscosity and measured in Krebs unit (KU).
Since associative thickeners contain hydrophobic groups, they have the capacity of forming intermolecular association through their hydrophobes. They can also adsorb onto the hydrophobic surfaces of dispersed particles, such as latex and pigments. As a result, the associative thickeners have a much greater thickening effect than the corresponding polymer having the same molecular weight with no hydrophobic groups. One method of solving the problem of high viscosity is known in the art. This involves dissolving the associating thickener in water and then adding the solution to the aqueous formulation. However, this approach restricts the amount of polymer that could be dissolved in a given amount of water without encountering very high viscosity. Another approach to prevent high viscosity buildup is to make a dispersion of the associative thickener normally with an organic solvent. However, this approach should be avoided since organic solvents are volatile and many of them can cause environmental damage. Yet another approach to reduce viscosity of aqueous solutions of associative thickeners is to add nonionic surfactants to aqueous solutions of associative thickeners.
Several patents are known that disclose the making of pourable aqueous solutions or dispersions of associative thickeners using surfactants. U.S. Pat. No. 5,425,806 describes an aqueous dispersion of an associative thickener with reduced volatile organic compound (VOC) that is pourable at 25° C. The dispersion includes 15-40 weight percent of an associative thickener (i.e., polyurethanes, polyesters, modified cellulosics, polyester urethanes, polyether alpha-olefins, and polyether polyols), 30-85 weight percent of water, and 1-30 weight percent of one or more anionic or nonionic surfactants. U.S. Pat. No. 5,378,756 discloses a thickener composition of a polyurethane polymer, a nonionic emulsifier and a compound that contains an acetylenic group (a triple bond) in the middle. U.S. Pat. No. 6,150,445 discloses an aqueous concentrate of 10-50 weight percent of a polyurethane associative thickener and 1-25 percent of a nonionic surfactant containing ethyleneoxy and propyleneoxy groups. U.S. Pat. No. 6,063,857 discloses a composition of water, at least 5.0 weight percent of a neutralized hydrophobically modified alkali-soluble emulsion polymer and less than 0.5 weight percent of a surfactant. None of these prior art discloses the present invention.